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A decision that many thought would be simple in the wake of Sam Cane's hamstring injury may not be clear cut, with All Blacks coach Steve Hansen suggesting there is a bit of thinking to do around the vacant No 7 jersey.

Cane will be out of action for at least a month and in his absence, the door appears to be open for Ardie Savea to win his first test start with Matt Todd, who has joined the squad as a replacement, being a contender for a place on the bench against South Africa this week in Christchurch.

That's probably where the selectors will end up. Probably but not definitely as while Savea has become a crowd favourite through his electric performances with the Hurricanes, Hansen cautioned against making assumptions.

His wariness is driven by the knowledge that for all his enormous potential, Savea remains an athlete with a bit of ground to make up physically.

He has incredible speed and instincts which make him such an effective ball carrier and game breaker, but the core requirement of the All Blacks openside is to be relentlessly physical on defence; dominant in the tackle and capable of staying strong over the ball in the face of heavy traffic.

Against a large and imposing South African pack that lives for the confrontation, Savea will be taken to the limits of his physical ability.

"We had a good chat last week about how he's coming along," revealed Hansen. "He's happy and we are happy with the progress but we know there are some things that he can still do better.

"It is just the pace of the [international] game is more demanding. He's a real explosive athlete so is he ready to play 80 minutes? Not sure yet at that level but we will get a good 60 out of him and we know Matty [Todd] can play for as long as we need him to play. What we do know about Ardie is that he's a quality player and he's just going to get better in time."

It's sometimes hard to remember because he's been around for a few years and has played so well in Super Rugby, that Savea is only 22 and at 102kg, is some way lighter than most of the other loose forwards playing test football.

International opensides are subjected to the most extreme physical pounding and it can be psychologically daunting. There's nowhere to hide and the severity of that challenge makes it easy to underestimate the big difference between Super Rugby and test football.

The All Blacks want to keep developing Savea and expose him to tough opponents, but they don't want to ask too much of him too soon.

"Mentally it is a big jump and physically it is a big jump," says Hansen. "You are combining the best players in the country into one team and Ardie has still got weight to put on and strength to gain and just as importantly he's understanding how we play and that takes a little time for people to bed in.

"Some guys get it pretty quickly and some guys don't. With Sam playing the way he was, it was probably the best form of his career. But he's gone from being about 102kg when he first came in to about 108kg-109kg and that extra weight gives him a lot more strength and power and he's able to do things that others can't.

"Defensively he's been fantastic. I don't know if Ardie will get to that weight but he's certainly come a long way. When he started Super Rugby he was only 95kg. He's up to about 102kg now so if we can get him up to about 105kg-106kg which is where I think he will end up, that will be a good weight for him."